The focus of these studies is the regulation of transcription of the MDR1 gene. Through understanding how this clinically important gene is regulated, we may be able to modify its expression and potentially control the development of multidrug resistance.In fulfilling the Specific Aims we have made significant progress in understanding the regulation of MDR1 transcription through study of promoter function in vitro and in vivo. We have defined MDR1 promoter sequences that are sufficient to confer basal and regulated transcription and have provided the first demonstration of specific cellular mediators of MDR1 gene regulation. Using the knowledge gained from our previous studies,we propose to examine the regulation of MDR1 transcription in hematopoietic development and disease.We have chosen to study MDR1 transcription in this tissue because 1) the MDR1 gene is expressed in hematopoietic progenitor cells as well as normal peripheral blood lymphocytes and monocytes, 2) increased MDR1 gene expression has been associated with treatment failure in tumors from this tissue, 3) TPA and chemotherapeutic drugs activate the MDR1 gene in hematopoietic cell lines, and 4) TPA activates the MDR1 promoter in K562 erythroleukemia cells. Finally, analysis of MDR1 gene expression is facilitated by the relatively homogeneous nature of hematopoietic tumors. Our hypothesis is that the MDR1 gene is transcriptionally regulated through normal signal transduction pathways in hematopoietic cell development and disease. Specific Aim 1 is to define the cis-acting MDR1 sequences and trans-acting cellular mediators of transcriptional activation of the MDR1 gene in hematopoietic cells. The results of these studies will provide insight into the specific signalling pathways controlling MDR1 gene expression in hematopoietic cells. In Specific Aim 2, we will examine Ras activation of MDRI transcription and determine if Ras mediates TPA- activation of the MDR1 promoter in hematopoietic cells. Recent data indicate that the proto-oncogene product Ras, which act as a control point in signal transduction pathways, activates the MDR1 promoter in transfected cells. We hypothesize that activation of this proto-oncogene may result in increased levels of MDR1 expression in human tumors. In Specific Aim 3 we will look directly in human acute myeloid leukemia cells to determine if activation of signaling pathways, through activation of Ras correlates with expression of the MDR1 gene. By looking in human tumor cells we can directly test the hypothesis derived from our studies in transfected cells. The knowledge gained from the proposed experiments will help to identify individual steps in the control of MDR1 gene expression in hematopoietic cells. Identification of cellular mediators which control MDR1 gene expression may provide targets for blocking activation of MDR1, and, thus, blocking the development of multidrug resistance.